Metal ion replenishment techniques of this type are known in the art. One typical technique is disclosed in Japanese Patent Application Kokai No. 171699/1982 as comprising immersing one metal to be plated and another metal having a nobler standard electrode potential than the one metal in the plating bath and electrically coupling them, thereby dissolving the one metal into the bath as an ion in accordance with the principle of electrochemical cell. This technique uses platinum, gold or a similar metal element as the other metal having a nobler standard electrode potential. We found that the use of such a noble metal element electrode as the counter electrode is not fully effective in practice because of a slow rate of dissolution of metal from the soluble electrode.
In order to increase the rate of dissolution of metal from the soluble electrode, we found that the dissolution rate can be increased by a factor of 2 or more by using an electrode having a platinum group metal oxide on a surface as the counter electrode. We then proposed in Japanese Patent Application No. 318296/1989 a new method for replenishing metal ion to a plating bath in accordance with the principle of an electrochemical cell in which an electrode having a platinum group metal oxide on a surface is used as the counter electrode to the soluble electrode.
Higher rates of replenishment of metal ion to the plating bath provide many advantages, including the reduced volume of a dissolving tank. Therefore, there is a desire for further increasing the rate of replenishment of metal ion, that is, the rate of dissolution of metal ion from the soluble electrode.